Maryland Sea Grant: Blue Crab Stock Enhancement Lessons

Japanese Hatchery-based Stock Enhancement:
Lessons for the Chesapeake Bay Blue Crab

Table 2. Scenarios of Chesapeake Bay stock supplementation for a depressed stock of blue crabs (harvest level 100 million individuals), based upon survival of released juveniles and harvest rates. Post-release survival of C4 crabs was estimated according to Miller (2001) who used Peterson and Wroblewski's (1984) size-dependent mortality model to estimate post-settlement and juvenile mortality rates. The C1 - C4 period must be decremented from this mortality rate. Because the duration of this period in the wild is uncertain (in hatcheries, the duration is less than 30 days), three duration levels were evaluated. Harvest rate is estimated by equation (1) where F = 60% yr-1, M = adult natural mortality (45% yr-1), and t=mean period for which adults are vulnerable to exploitation (1 yr) (Gulland 1983). Harvest rate parameter estimates are from CBSAC stock assessment (CBSAC 2001) and other demographic studies (Ju et al. 1999). Harvest Recapture Rate represents the probability of harvesting a released crab and is the product of Post-release Survival and Harvest Rate. A depressed Chesapeake Bay Landings level is stipulated at 100 million individuals. During the period 1970-1999, mean landed number of Chesapeake Bay blue crabs was 190ą 40 (s.d.) million.


Post-release Survival C-4 - Recruitment to Fishery			Harvest Rate	Harvest Recapture Rate	"Depressed" Ches. Bay Landings (no.)	C-4 Release no. for 10% Supplement

Size-dependent mortality	30 d	0.45%	47%	0.21%	100,000,000	4.8 x 10⁹
model (Miller 2001):	40 d	1.0%	47%	0.47%	100,000,000	2.1 x 10⁹
30-60 d C-1 - C-4 period	60 d	5.0%	47%	2.35%	100,000,000	4.3 x 10⁸

Hamana Lake harvest recapture rate (Okamoto 2001)				1%	100,000,000	1.0 x 10⁹
Okayama Prefectural harvest recapture rate (Kurakawa 2001)				6%	100,000,000	1.7 x 10⁸

– David Secor